Use of ladar technologies to form images of targets is set forth in U.S. Pat. Nos. 5,608,514 and 5,877,851 assigned to the same entity, and herein incorporated by reference.
Ladar is basically a radar transformed from a system for measuring reflected radio frequency electromagnetic radiation, typically in the 1-25 cm wavelength range to locate remote objects, to a system that operates on laser radiation, typically in the 0.1 to 10.0 μm range. The resulting combination of radar and optics provides a system with inherently enhanced accuracy in the measurement of range, velocity, and angular displacement. Moreover, the high carrier frequency allows ladar systems to be made more compact in physical dimension, which is particularly attractive in aircraft, projectile, space and other volume-limited applications.
U.S. Pat. No. 5,608,514, (the '514 patent) issued Mar. 4, 1997 describes a technique for achieving high range resolution for a single pixel ladar by employing frequency modulation (FM) radar ranging principles.
Three-dimensional imaging of a scene is achieved by mechanically scanning the single pixel ladar or by building arrays of such ladars. To perform ranging on a single pixel, the ladar's laser transmitter is amplitude modulated with a radio-frequency subcarrier which itself is linearly frequency modulated. The target-reflected light is incoherently detected with a photodiode and converted into a voltage waveform.
The voltage waveform is then mixed with an undelayed sample of the original laser amplitude modulated waveform. The output of the mixer is processed to remove “self clutter” that is commonly generated in FM ranging systems and obscures the true target signals. The clutter-free mixer output is then Fourier transformed to recover target range.
The '514 patent teaches the use of a single pixel ladar and the use of a mechanical scanner to scan the laser output beam and the receiver or detected field of view to form an electronic image of the target. The development of a scannerless ladar generally requires building a receiver based-on some variation of focal plane array technology (FPA) such as used in conventional imaging cameras.
Another receiver design was conceived and disclosed in U.S. Pat. No. 5,877,851 (the '851 patent) where an electro-optic light modulator, positioned in the receiver light path, performs demodulation optically and a standard focal plane array performs detection of the demodulated light. Thus, while the laser transmitter is modulated as in the '514 patent, the entire scene or field of view is illuminated.
The method and apparatus of the '851 patent may be applied to both one-dimensional and two-dimensional detector arrays having any number of detectors. Numerous image frames are recorded periodically in time over a frequency modulation (FM) period. A Fourier transform taken over the time period for a pixel establishes the range to the target in that pixel. Performing the Fourier transform for all pixels yields a three-dimensional image of objects in the field of view. Using a focal plane detector array, electro-optical light modulator, and microwave circuit elements in conjunction with frequency modulated continuous wave (FM−/cw) radar ranging theory yields a scannerless ladar possessing high range resolution without range ambiguities.
While the ladar system of the '851 patent is scannerless, there are limitations. One limitation is that a separate electro-optic light modulator and a separate focal plane array are used to demodulate and detect the return or received light signal from the target. Further, the ladar approach of the '851 patent normally uses of a large area (i.e., 4×4 mm) QWEO modulator that is driven with the ladar's local oscillator voltage over the required bandwidth. The capacitive load presented by such a device is in the low 1000's of picofarads which may be difficult to drive using microwave amplifiers. The bandgap of the QWEO modulator and the laser line must remain coincident over temperature variations which adds to system complexity.